The present invention relates to a thermal transfer (printing) material, and more in detail, the present invention relates to a thermal transfer (printing) material comprising, as a base material, a biaxially oriented polyethylene 2,6-naphthalate film which has a high Young's modulus in the longitudinal direction and heat-resistance, is able to thin the thermal transfer (printing) material and gives clear transcribed letters, and a thermal transfer layer coated on the film.
In recent years, along with the development of office automation, various printing systems have been developed, and of them, a thermal transfer printing system which is small in noise in printing and simple in operation has attracted one's attention.
Ordinarily, a heat-sensitive printing system wherein a thermal head is directly brought into contact to a heat-sensitive printing paper containing a specific colour former has been used. However, although the above-mentioned system is excellent in the operability and maintenance of the thermal printing apparatus, there is a large problem in the long-term preservation of the thus prepared records, because the printing paper is apt to be discolored by heat and light.
Accordingly, a thermal transfer printing system has been spotlighted recently as a system which utilizes the merit of the heat-sensitive printing system and removes the above-mentioned demerit, and has been used broadly.
It is considered that the above-mentioned thermal transfer printing system will be used in future in various printers because of the possibility of printing on ordinary paper sheets, of the excellence in longterm preservation of records and of the excellence in quality of the printed letters. As the printers, for instance, printers for computors, printers for word-processors, video-printers, labelprinters, printers for stillvideocamera, facsimile, etc. may be mentioned. Furthermore, with the remarkable development of computors which easily carry out colorgraphic representation, the improvement of the printer is effected for coloration.
In the thermal transfer printing system, a thermal transfer (printing) material comprising a base material and a thermal transfer layer coated on the base material is used. In case of printing letters, a recording paper sheet (an ordinary paper sheet can be used) is brought into contact to the thermal transfer layer of the thermal transfer (printing) material, a pressure is applied thereon, a thermal head is brought into contact to the opposite side of the base material, a signal current is transmitted to the thermal head in pulse wise to heat the thermal head.
By such a way of transferring a heat-melting ink only from the heated part of the thermal transfer layer or subliming a sublimable ink therefrom, transcription is carried out onto the recording paper. In ordinary cases, the heat-melting ink is easily transferred on the recording paper in the temperature of from 60 to 120.degree. C. of the temperature of the ink, and the sublimable ink sublimes at a temperature of the ink in a range of from 70 to 200.degree. C. and is transcribed.
Generally, the method utilizing the heat-melting ink is called the heat-melting process of thermal transfer printing system, and the method utilizing the sublimable ink is called the subliming process of thermal transfer printing system.
In the cases of the heat-melting process of thermal transfer printing system, a thermal transfer (printing) material having a thermal transfer layer containing carbon black is used in the case of printing all in black color, and for coloration, after preparing separately the three thermal transfer (printing) materials respectively having the thermal transfer layer of one of the three primary colours, namely yellow, magenta and cyan, each colour is thermally transferred once on the same recording paper sheet, three times in total, thereby piling up the three primary colours on the same recording paper sheet.
On the other hand, a characteristic of the subliming process of thermal transfer printing system lies that the harmony of colour tone is easily effected, namely the easy control of the amount of subliming ink is easily effected by the amount of heat of the thermal head.
For making the colour deeper, the applied voltage is raised or the applied time of current pulse is elongated, and on the other hand, for lightening the colour, the applied voltage is lowered or the applied time of current pulse is shortened. By utilizing the above-mentioned system of easily effecting the harmony of colour tone, the subliming process of thermal transfer printing system is applicable in colour print, particularly in colour print of detailed figures and photographs. In the system of colour printing, in the same way as the heat-melting process, the three thermal transfer (printing) materials respectively having one of the three thermal transfer layers of the three primary colours, namely yellow, magenta and cyan are prepared, and the thermal transferring is effected once in one colour, 3 times in total. In the case of using the subliming process of thermal transfer printing system, it is necessary to raise the amount of heat in the thermal head in order to make the colour deeper and accordingly, a higher temperature is applied on the thermal transfer printing material than in the case of using the heat-melting process.
Although in the case of using a subliming ink in the thermal transfer printing system, a special image-receiving layer may be provided on the recording paper sheet, in general, since the recording on an ordinary paper sheet by the thermal transfer printing system is easily carried out, it is possible to remove the demerit of the conventional heat-sensitive recording system. Namely, the thermal transfer printing system is an excellent recording system.
Hitherto, as the base material for the thermal transfer (printing) material, a condenser paper has been used. However, due to the reasons that the condenser paper is low in strength and is easily teared off, and that in the case where it is necessary to thin the base material for obtaining a high speed printing and a clear image, it is difficult to obtain a thinner condenser paper or to reduce the thickness irregularity of the condenser paper, a thin film of polyethylene terephthalate is now used as the base material.
However, at present, for obtaining a further clearer image, a thinner film having a higher heat-resistance is demanded.
However, the polyethylene terephthalate film is insufficient in heat-resistance, and a thin polyethylene terephthalate film having the Young's modulus of not less than 600 kg/cm.sup.2 can not be produced. In addition, the polyethylene terephthalate film has a defect of reducing the strength in transverse direction in the case of raising the strength in longitudinal direction.
Accordingly, a development of a film which is more heat-resistant than polyethylene terephthalate film and has a higher strength in longitudinal direction than polyethylene terephthalate film has been demanded. Further, a development of a film which has a higher strength in longitudinal direction than the film of polyethylene terephthalate and at the same time, a high strength in transverse direction has been demanded.
On the other hand, a biaxially oriented polyethylene 2,6-naphthalate film which has the Young's modulus of not less than 510 kg/mm2 (51000 kg/cm.sup.2) in longitudinal direction and the Young's modulus of not less than 680,kg/mm (68000 kg/cm.sup.2) in transverse direction is useful as the electric insurating material, the supporting material of magnetic recording tape etc. [refer to Japanese Patent Application Laying-Open (KOKAI) No. 50-45877(1975)]; and a magnetic recording tape prepared by forming a magnetic layer on the surface of a biaxially oriented polyethylene 2,6-naphthalate film having the Young's modulus of not less than 510 kg/mm.sup.2 in longitudinal direction and the Young's modulus of not less than 680 kg/mm.sup.2 in transverse direction [refer to Japanese Patent Publication No. 56-19012(1981)]have been proposed. However, there are no description nor suggestion concerning the use of the polyethylene naphthalate film as the base film for thermal transfer (printing) material.
As a result of the present inventor's studies, it has been found by the present inventor that the abovementioned problems can be solved by using a polyethylene 2,6-naphthalate film of the specified Young's modulus instead of the polyethylene terephthalate film, and on the basis of his finding, the present inventor has attained the present invention.